The present invention relates to a method for producing a draining channel at a support structure, and to an apparatus for producing such a draining channel. More particularly, the present invention relates to a method and apparatus for draining tunnels or similar underground excavations.
There are many instances in which, for instance, water is to be drained from underneath or behind a fluidimpermeable layer, such as a concrete wall or the like. In such situations, it is usually necessary to provide a plurality of draining channels behind or underneath the layer, which draining channels collect the water and conduct the same to a spaced location at which it is discharged. The reason for providing the draining channels is to prevent build-up of pressure underneath or behind the layer, which would otherwise result in damage to the layer. In the following discussion, the present invention will be explained as utilized in underground excavations, such as tunnels, in which the above-mentioned layer is a concrete wall structure lining the natural walls of the excavation. However, this is not the only application to which the method and apparatus of the present invention can be put, as will be readily appreciated.
During the construction of tunnels or other underground spaces, it is a very frequent occurrence that water seeps from the natural rock or soil surrounding the underground space or tunnel into the latter, and such water must, of course, be conducted or pumped away from such an underground space. When the rock surrounding the underground space is kept in its natural state, except for the provision of spaced supports, the water seepage usually does not create any problems; however, the situation is different when the rock is to be provided with a concrete lining which gives the tunnel or underground space its final shape. Under these circumstances, the water which enters the space to be lined to form droplets or even springs, renders the application of the concrete lining to the natural rock wall more difficult, or even impossible. Most of the conventional methods of applying the concrete lining to the natural rock bounding the underground space are based on the assumption that the natural rock bounding the excavation is relatively dry and stable, inasmuch as the presence of water streams, or even small dislocations of the natural rock which may be caused by water-pressure build-up, would result in destruction of the concrete applied to the natural rock to forming a lining thereon prior to the hardening of the concrete, which would result in destruction of the final lining even before it is hardened.
In some circumstances, the quantity of water which enters the underground space may be substantial, and such situations will be further referred to as underground springs; in other circumstances, the quantity of the water is negligible, but the pressure thereof caused by the column of water between the water table above the excavation and the latter may be substantial. It will be appreciated that the substantial amount of water flowing from the underground spring will entrain and carry away with it the freshly applied concrete from the vicinity of the spring; on the other hand, the pressure of the water, no matter how neglibible its amount is, behind the freshly applied concrete will cause its dissociation from the natural rock wall and, over a period of time, damage to the concrete lining. These problems have already been recognized, and it has already been proposed to provide a plurality or a network of draining channels between the concrete layer and the natural rock so as to remove the accumulating water from between the same.
Of several methods of removing seepage water from between the natural rock and concrete lining, two which are most closely related to the present invention will now be discussed. In the first method, spray concrete or gunite, to which a setting accelerator is added, is gunned on the wall of the tunnel or excavation until its thickness is sufficient to prevent the water seeping through the natural rock from entering the tunnel. However, this method has a serious drawback; namely, even if the concrete sets or hardens so quickly that it cannot be carried away by the water seeping into the tunnel, the pressure build-up behind the layer of the hardened spray concrete or gunite can cause entire plates to separate from the remainder of the concrete layer and fall into the underground space, whereupon the water again flows into the underground space and the final layer of concrete cannot be applied over the gunite layer. On the other hand, even if the gunite layer is strong enough to withstand the pressure of the seepage water until the final concrete layer is applied over the gunite layer and hardens to form the lining of the underground space, the water which accumulates between the natural rock wall and the gunite layer, which water is at relatively high pressure, flows even through finest hair cracks in the gunite and concrete layers into the interior of the tunnel. On the other hand, this method is rather advantageous in that it can be easily performed; all that is necessary for performing this method is a spraying arrangement for applying a jet of gunite to the natural rock wall. Another advantage of this method is that it is not necessary to cover the entire tunnel wall of natural rock with the gunite, but rather only the region of the wall surrounding and including the place at which the water infiltrates or seeps into the tunnel. Thus, this method is rather advantageous and inexpensive in terms of labor and material, and can be used under all circumstances where the amount or pressure of the seeping water are rather low.
In a second conventional method, rigid elongated shells of asbestos cement or flexible synthetic plastic material, having annular segment cross sections, are placed over the natural rock tunnel wall in the region of water seepage, and are manually connected to the tunnel wall by means of a concrete with a setting accelerator admixed thereto. Where a plurality of such shells is so connected to the natural rock wall of the underground space, a draining conduit system is obtained which is capable of conducting the seeping water away from the point of seepage and to a discharging location. The above draining system, which is provisionally attached to the natural rock wall by means of the quick-setting concrete, is then additionally covered by a layer of spray concrete or gunite, such a layer having a sufficient thickness to prevent seepage of water therethrough and to securely attach the shells to the natural rock wall. It will be appreciated that, as a result of the provision of the draining system in the region of water seepage, the build-up of high water pressures behind the layer of the spray concrete, due to the seepage of water through the natural rock surrounding the excavation, is avoided. However, this method also possesses a very serious drawback, which is the necessity of placing and attaching the draining shells manually, which is an extremely laborious and expensive proposition.